1. Field of the Invention
The invention relates to a capillary optical element with a complex structure of capillaries and a method for its manufacture. The capillary optical element can be used, for example, in diffractometry, micro-diffractometry, x-ray fluorescent analysis, micro-x-ray fluorescent analysis, and in photoelectron spectroscopy. In all these cases, the light intensity can be increased by several orders of magnitude.
2. Description of the Related Art
Poly-capillary optics has emerged over the past decade as a new important field in x-ray optics. The underlying principle of poly-capillary optics is the channeling of x-rays through multiple total internal reflection on the interior walls of glass capillaries. Curved as well as straight capillaries conduct x-rays along their respective axes. In this way, the propagation direction of the x-rays can be directly influenced.
A bundle of specially formed glass capillaries which are combined into a monolithic structure forms a so-called poly-capillary lens. For example, a bundle of curved capillaries which have their axis on one end oriented towards an x-ray source and on the other end oriented towards an arbitrarily selected point, form a focusing lens, as shown schematically in FIG. 1.
The aperture angle at the input end of the lens can be between 0.1 to 0.3 radians (rad). Each capillary only captures a limited solid angle which is determined by twice the critical angle xcfx86c of the outer total reflection and inversely proportional to the energy. For example, for an energy of 10 keV the critical angle xcfx86c is only 6 mrad. Accordingly, the linear dimensions of the captured source SS and of the focal point SF can be determined by the following formulas:
For the captured source dimension:
SS=(d2+(fS2xcfx86c)2)1/2
and for the size of the focal spot at focus:
SF=(d2+(fF2xcfx86c)2)1/2
wherein d is the diameter of a capillary, fS and fF are the focal lengths of the lens on the side of the source and the focus, respectively. The critical angle for total reflection xcfx86c depends on the x-ray energy and the material of the capillaries.
Accordingly, the optimal dimensions of the source as well as the size of the focus spot depend on the x-ray energy and on the focal lengths at the input and output side of the system. According to the relationships given above, the size of the source and of the focused radiation beam can be varied over a wide range, for example, from 10 xcexcm to several millimeters, by varying the form and the size of the capillary lens. If the capillary lens is divided in the center and only one half of the lens is used, the capillaries are parallel to each other on one side of the divided lens and can produce a quasi-parallel x-ray beam. Such system is referred to as a xe2x80x9chalf lens.xe2x80x9d
It has been observed that radiation originating from a source can be captured very effectively over a specified range of solid angles by a lens and converted either into a parallel beam or focussed to a small spot. For example, the x-ray beam can be focused by a full lens to a focal spot, where the intensity of the focused x-ray beam at the same distance from the source is several orders of magnitude greater than without a lens.
A poly-capillary lens with capillaries of a certain size is transparent over a wide range of energies from a fraction of a keV to several tens of keV. However, the lenses have a different efficiency at different energies, so that the transparency for x-rays increases over a certain energy range and at the same time decreases over other energy ranges. The underlying reason for this phenomenon is that the functionality of a curved capillary is determined by two factors. The first factor is a coefficient xcex3 which indicates how much of the radiation incident on the capillary is captured and is given by the formula xcex3=xcfx86c2R/d. R is here the bending radius of the capillary and d the diameter of the capillary. The coefficient xcex3 has an energy dependence which causes xcex3 to decrease steeply above a certain value of a critical angle xcfx86c=(R/d)xe2x88x921/2(xcex3=1). Accordingly, capillaries with a smaller diameter are required for channeling x-rays of higher energy. The second factor that influences the functionality of a curved capillary is the transmission coefficient of the capillaries, which is related to the attenuation of the x-rays passing through the capillaries. This factor has a more complex dependence on the energy. The transmission of the low-energy portion of the radiation decreases with decreasing diameter of the capillaries. This is related to the increase in the number of reflections, in particular at low energy, because the critical angle is greater and the reflection coefficient is smaller at lower energy than at higher energy.
A possible remedy for improving the transmission efficiency related to the first factorxe2x80x94which is associated with the coefficient xcex3 that describes the capture of the radiation incident on the capillaryxe2x80x94is disclosed in U.S. Pat. No. 5,745,547. However, the solution proposed therein has the disadvantage that the transmission efficiency is enhanced only over a relatively narrow energy range. Many practical application, however, require a more or less uniform transmission over a wide energy range.
However, if both of the factors mentioned above and in particular their interaction are taken into consideration, then the following result is obtained: given the specific diameter of the capillaries, there exists an optimal energy range at which x-ray radiation passes through the capillaries with the highest efficiency. When the diameter of the capillaries is reduced, the optimal energy moves to higher energy values while the transmission at lower energies simultaneously decreases. Conversely, capillaries with a larger diameter show a better transmission efficiency at lower energies. For example, capillaries with a relatively large diameter of approximately 100 xcexcm transmit radiation optimally at keV energies, whereas the intensity of the higher energetic components is strongly reduced. Capillaries with a diameter of approximately 5 xcexcm optimally transmit radiation with an energy of approximately 10 keV. Accordingly, the transmission efficiency for a specified capillary diameter decreases both above and below the optimal energy.
Most practical applications of capillary lenses require a more or less uniform transmission over a broad energy range. For example, x-ray fluorescent analysis requires excitation over the greatest possible range of elements (from sodium to uranium), which can be achieved with a radiation spectrum ranging from several keV to 30 keV. Another example is diffractometry, where depending on the measured material, characteristic radiation of the elements from chromium to silver is required. It would therefore be desirable to employ the same x-ray optics over the entire energy range from 4 keV to 25 keV when changing the anode material (which can be easily changed with a suitable design of the x-ray source that does not require exposing the tube to air).
Another disadvantageous effect, aside from the effect due to inhomogeneous transmission, results from a direct transmission of high-energy radiation through the lens as a whole and through the thin walls of the capillaries. This effect causes a washed-out halo of high-energy radiation (halo effect) in addition to the sharp focal spot in the focal plane from the focussed lower energy beams. This undesirable side effect makes it difficult, for example in x-ray fluorescent analysis, to unambiguously localize the heavy elements whose characteristic lines are excited by high-energy radiation.
It is an object of the invention to provide a method and a device which enables poly-capillary lenses to transmit uniformly over a wide energy range and which also reduces the direct transmission of high-energy radiation through the lens as a whole and through the thin walls of the capillaries.
According to a particularly advantageous embodiment of the invention, the capillary optical element is made of a complex structure of capillaries, wherein the diameter of the capillaries 12 in the core 6 of the element 1 is smaller than the diameter of the capillaries 10 in the peripheral region 4 of the element 1.
A method for producing capillary optical elements advantageously includes in a first step of drawing capillaries with different diameters and subsequently forming an capillary optical element 1 by arranging the capillaries in such a way that the capillaries with the greatest diameter enclose on the outside those capillaries with the smallest diameter that are located in the interior region of the element 1.
Other objects and features of the present invention will become apparent from the following detailed description considered in conjunction with the accompanying drawings. It is to be understood, however, that the drawings are intended solely for purposes of illustration and not as a definition of the limits of the invention, for which reference should be made to the appended claims.